The present invention relates to material fatigue testing and particularly to an apparatus for maintaining a test specimen at a control temperature and for cyclically inducing a range of temperatures between cryogenic and elevated.
Spacecraft endure a wide range of temperatures during space flights. The portion of the spacecraft exposed to the intense direct rays of the sun experience elevated temperatures while the portion of the spacecraft shaded from the sun experience relatively lower temperatures. Moreover, the spacecraft as a whole experiences low temperatures during travel through the shadow of a planet or other heavenly body. The temperatures vary from cryogenic, hundreds of degrees below 0.degree. centigrade, to elevated, hundreds of degrees above 0.degree. centigrade. Furthermore, the temperature of a particular spacecraft structural component may experience a wide range of temperatures in a short period of time during rotation of the spacecraft, in that the particular component is alternately exposed to the sun's rays and shaded therefrom.
In the testing of structural components of spacecraft it is desirable to simulate as closely as possible the pertinent environmental conditions the structure will experience in space. Accordingly, in the fatigue testing of structural components of spacecraft it is desirable to expose the structure to extreme high and low temperatures while maintaining a load on the test specimen. Relatively massive commercial systems are available in the prior art either as heat sources or as cryogenic sources but none include cyclic temperature capabilities into the cryogenic temperature range. In some known systems test specimens are alternately transported between a cryogenic chamber and a hot chamber. However, these systems are unacceptable for tests in which the specimen position must remain fixed as in a testing machine.
The massive commercial systems of the prior art are incompatible with the fatigue testing machine designed to test small samples. Furthermore, they require large amounts of electricity for heating, large amounts of cryogenic material for cooling, and do not provide cyclic temperature capability. The systems which transport this space specimen between hot and cold chambers are inherently incompatible with the stationary configuration of a fatigue testing machine.
The present invention alleviates to a great extent the short comings of the prior art. In the present invention the same block having a small mass is used to transfer both elevated and cryogenic temperatures to the test sample. The block is clamped to the test specimen during fatigue testing. Liquid nitrogen is directed through the block for cooling it to cryogenic temperatures.
Heating cartridges accommodated by apertures in the block, are provided for heating the block to elevated temperatures. A thermocouple probe penetrates the block to monitor the temperature of the test specimen. Control devices are provided to supply liquid nitrogen through the block and for power to the heating cartridges in a manner to maintain a preselected temperature. Cyclic operation between elevated and cryogenic temperatures with hot and cold cycles of unequal lengths, and at arbitrary times is possible by utilizing a timing device to alternate between heating and cooling. The small mass of the block is the key feature of the invention because its temperature can be changed rapidly allowing quick heating and cooling of the test specimen.
An object of the invention is to provide an apparatus for heating and cooling a fatigue test specimen.
It is another object of the invention to provide a heating and cooling apparatus having a minimal mass to allow quick cooling and heating thereof.
A further object of the invention is a heating and cooling apparatus having the foregoing advantages and which is capable of cyclic operation between cryogenic and very hot temperature.
Another object of the invention is a heating and cooling apparatus having the foregoing advantages which can be easily custom made to fit fatigue test samples connected in place to a testing machine.
A further object of the invention is a heating and cooling apparatus having the foregoing advantages which is inexpensive and uses small amounts of energy to heat, and cryogenic materials to cool.
Other objects and advantages of the present invention will be readily apparent from the following description and drawings which illustrate the preferred embodiment of the invention.